Transmitting device



June 12, 1934. w BROWN 1,962,294

TRANSMITTING DEVICE Filed March 27, 1928 Fig: 1.

INVENTOR Walter J. Brown AT'TORNEY Patented June 12, 1934 UNITED STATES PATENT OFFICE TRANSMITTING DEVICE Application March 27, 1928, Serial No. 265,005 In Great Britain December 30, 1927 1 Claim.

My inventionrelates to amplifying systems for high-frequency currents, and it has particular relation to amplifying systems of the type wherein a tunable oscillatory circuit is included in either the input or the output circuit of a multielectrode thermionic device, and a portion of said oscillatory circuit is utilized to neutralize the system against parasitic oscillations.

It has heretofore been recognized that the inter-electrode capacities of thermionic devices provide coupling paths between the input and output circuits thereof, whereby sufficient energy may be fed back to the input circuit to cause the generation of undesired oscillations. Many systems have been devised to counteract the coupling effect of theinter-electrode capacity, none of which, however, have been entirely satisfac- "tory over the entire tuning range of the amplifier.

The majority of schemes employed to obtain neutralization of inter-electrode feed-back, provide a parallel feed-back path through a 'coil and a neutralizing condenser, the coil being connected in reverse phase to a similar coil through which the inter-electrode feed-back energy passes. To efficiently neutralize, over a wide range of frequencies, the energy normally fed back through the inter-electrode capacity of the thermionic devices employed in the circuit, it is.essential that the coils be maintained in a balanced condition; that is, the potential drop across the one coil must be substantially the same as that across the other throughout the frequency range of desired neutralization. While the coils may be sufficiently matched as to physical and electrical dimensions, it has been found that, when employed in a circuit as described above, the potential drops across the coils do not match up but, on the contrary, are unbalanced and become more so as the frequency of the'current therethrough increases. This is an inherent characteristic of the circuits and is brought about by reason of the fact that the capacities to ground of the coils differ, thereby providing bypass paths to ground of different values. This changes the apparent impedance of the coils whereby the potential drops across them will differ. This inherently unbalanced condition may further be aggravated by a difference in capacity to ground of the anode of one tube and its connection and the grid of the following tube and its connection, these capacities to earth also providing bypass paths of unequal value for the high-frequency current in the system.

According to my invention, I have overcome this inherent undesirable characteristic existing in radio circuits, whereby to establish a condition of such character as to enable feed-back in the circuits to be more easily and efliciently neutralized over a wider range of frequencies. This result I have been able to accomplish by balancing the capacity to ground of one coil and its associated elements with the capacity to ground of the other coil and its associated elements, thereby balancing the capacities to ground about a point of zero potential. In this manner, the potential drop across each coil may be maintained of substantially like value.

One object of my invention, therefore, is to provide an amplifying system that shall be effectually neutralized against parasitic oscillations, over a wide tuning range.

In one preferred embodiment of my invention, as applied to an amplifier of the aforementioned type, a capacity is so arranged between a point on the oscillatory circuit and a point at a steady or earth potential, that the effective capacities to earth of the parts of the oscillatory circuit and other apparatus connected thereto, such as the electrodes of the thermionic device, etc., are symmetrically arranged about the said point on the oscillatory circuit, insofar as high-frequency currents are concerned. By the provision of the said constant-potential point, the electrostatic coupling between the balanced portions of the system is greatly reduced, and the tendency toward oscillation is much more easily compensated.

Among the novel features of my invention are those particularly set forth in the appended claim. The invention, in its generic aspect, however, as well as further objects and advantages thereof, will best be understood by reference to the following description of certain specific embodiments thereof, taken in connection with the accompanying drawing, in which Figure 1 is a diagrammatic view of a portion of an amplifying system comprising a preferred embodiment of my invention, and

Figs. 2 to 5, inclusive, are diagrammatic views of portions of amplifying systems comprising alternative forms of my invention.

Like reference numerals indicate corresponding parts in the several figures of the drawing, wherever applicable.

That portion of an amplifying system illustrated in Fig. 1 comprises a plurality of threeelectrode thermionic valves 1 and 2. The filaments of the valves are supplied with power from a common source (not shown). The anode of the thermionic device 1 is connected, through a high-frequency choke coil 3, to a source 4 of high-voltage direct current, such as a battery, a battery eliminator or the like. The anode of the valve 1 is also connected, through a blocking condenser 5, to the end 6 of an inductor 7, the said inductor having a tuning condenser 3 connected in shunt thereto. The filaments have a ground-connection 9. A point 16 on the inductor '7 is connected to a conductor 11 which extends between the cathodes of the several thermionic devices 1 and 2, and, in this connection, may be included a grid-bias battery or a grid-leak and condenser (not shown). A point 12 on the inductor 7 is connected, through a balancing condenser 13, to the grid of the valve 1. The end 14 of the inductor, opposite the end thereof to which the anode of the device 1 is connected, is connected to the grid of the second thermionic device 2.

In the circuit arrangement so far described, the effective capacities of the parts of, and the parts associated with, the tunable oscillatory circuit comprising the inductor 7 and the condenser 8, are unsymmetrical about the point 10, which point is maintained at earth potential, mainly by reason of the relatively large capacity to earth of the control electrode of the valve 2, which capacity is connected in shunt to a relatively large amount of the inductor '7. Also, in general, the capacity to earth of that part of the inductor which is included between the points 10 and 14 is greater than the capacity to earth of the part which is included between the points 10 and 6. On account of the dissymmetry of capacity to earth just referred to, the value of current which flows in the part of the inductor 7 between the points 10 and 12 is greater than that flowing in the part of the inductor included between the points 10 and 6.

The potential difference between the points 10 and 12 is, therefore, increased above the value it would have if the capacities of the system were symmetrical and, consequently, in order to effect a true balance between the energy fed back to the control electrode of the valve 1 through the capacity existing between the said electrode and the anode, and the amount of energy fed back by way of the balancing condenser 13, the size of the condenser 13 must be less than its theoretical value. Moreover, theratio of the current in the part of the inductor included between the points 10 and 12 to that in the part of the inductor included between the points 10 and 6 is not constant, but becomes greater as the capacity of the tuning condenser 8 is reduced, since the capacity between the cathode and grid of the valve 2 then constitutes a larger proportion of the total capacity of the oscillatory circuit. From this consideration, it appears that the balancing condenser 13 requires re-adjustment, in order to effect perfect balance, whenever the tuning condenser 8 is adjusted.

my invention, I have provided means whereby this diificulty is obviated. The said means comprises a condenser 15, preferably a variable condenser which is connected across the points 10 and 6 and is so adjusted that the current flowing in the part of the inductor included between the points 10 and 12 is substantially equal to the current flowing in the part of the inductor included between the points 10 and 6, whereby the effective capacities of the parts of, and those associated with, the oscillatory circuit are rendered symmetrical about the point 10 which is at earth potential.

In a practical application of my invention, the parts of the inductor included between the points 10 and 12, and the points 10 and 6, respectively, may be made equal, and the condenser 13 may conveniently be replaced by a thermionic device similar to the thermionic device 1, the grid and anode connections only of said device being utilized. In such an arrangement, it is convenient to also utilize the adjustaable condenser 15 which, by adjustment, may be arranged to compensate for any small errors in balance between the valve 1 and the valve utilized instead of the condenser 13.

Moreover, the inductor 7 may be constructed of a number of parts instead of as a continuous coil, as is illustrated in the drawing.

Figs. 2, 3, 4 and 5 are explanatory of alternative embodiments of my invention.

In the arrangement shown in Fig. 2, a condenser 20 is utilized to balance the capacity between the anode and the grid of the device 1, and a condenser 21 is used mainly to balance the difference between the capacity to earth of the grid of the valve 2 and the capacity to earth of the anode of the valve 1.

In the arrangement shown in Fig. 3, a condenser 30 is utilized to balance the capacity between the grid and the anode of the valve 1, while a condenser 31 is utilized mainly to balance the sum of the capacities to earth of the grid of the valve 2 and the anode of the valve 1.

Fig. 4 illustrates a modified form of amplifier which, in principle, is the same as those that have previously been discussed. In this arrangement, a condenser 40 is utilized to balance the capacity between the grid and the anode of the second valve 2, the balance being effected by means of the part of tuned input circuit which is formed by the condenser 8 and the inductor 7. A condenser 41 is utilized mainly to balance the capacities to earth of the grid of the valve 2 and the anode of the valve 1.

In the arrangement shown in Fig. 5, a condenser 50 is utilized to balance the capacity between the anode and the grid of the valve 2, and a condenser 51 is utilized to balance the difference between the capacity to earth of the grid of the valve 2 and the capacity to earth of the anode of the valve 1.

I have, accordingly, by my invention, provided novel means for neutralizing an amplifier system against self-oscillation, my invention being particularly applicable to amplifiers of the type wherein inter-valve coupling is accomplished through a single tuning circuit common to the output of a preceding valve and the input of the valve following.

My invention is mainly advantageous in that, by its practice, parasitic oscillations are positively prevented over a wide tuning range, and the quality of the reproduced signals is materially improved. Although I have illustrated and described only a few specific embodiments of my invention, numerous modifications thereof will be apparent to those skilled in the art. My invention, therefore, is not to be limited except insofar as is necessitated by the prior art or by the spirit of the appended claim.

I claim as my invention:

In an amplifying system, a plurality of valves. an output circuit associated with one of said for balancing the capacity to ground of the two coil elements defined by the aforementioned connections thereto, said means comprising a capacitor inserted between the anode connection to said coil and said grounded point.

WALTER JOHN BROWN. 

